The invention relates to cathode materials for Li-ion batteries, having the general formula LiaNixCoyMnzM′mO2±eAf (M′ being either one or more of Al, Mg, Ti, Cr, V, Fe, Ga; and A being either one or more of F, C, Cl, S, Zr, Ba, Y, Ca, B, Sn, Sb, Na and Zn) with a non-homogeneous Ni/Mn ratio in the particles, and having superior capacity, rate and fade rate characteristics. Also a method to manufacture these materials is disclosed.
Due to their high energy density, rechargeable lithium and lithium-ion batteries can be used in a variety of portable electronics applications, such as cellular phones, laptop computers, digital cameras and video cameras. Commercially available lithium-ion batteries typically consist of graphite-based anode and LiCoO2-based cathode materials. However, LiCoO2-based cathode materials are expensive and typically have a relatively low capacity of approximately 150 mAh/g.
Alternatives to LiCoO2-based cathode materials include LNMCO type cathode materials. LNMCO means lithium-nickel-manganese-cobalt-oxide. The composition is LiMO2 or Li1+x′M1-x′O2 where M=NixCoyMnzM′m. LNMCO has a similar layered crystal structure as LiCoO2 (space group r-3m). The advantage of LNMCO cathodes is the much lower raw material price of the composition M versus Co. The addition of Ni gives an increase in discharge capacity, but is limited by a decreasing thermal stability when increasing the Ni content. In order to compensate for this problem, Mn is added in the structure as a stabilizing element, but at the same time some capacity is lost.
The preparation of LNMCO is in most cases more complex than LiCoO2, because special precursors are needed wherein the transition metal cations are well mixed. Typical precursors are mixed transition metal hydroxides, oxyhydroxides or carbonates. Typical cathode materials include compositions having a formula LiNi0.5Mn0.3Co0.2O2, LiNi0.6Mn0.2Co0.2O2 or Li1.05M0.95O2, with M=Ni1/3Mn1/3Co1/3O2. Compared with LiClO2, LNMCO tends to have a lower bulk diffusion rate of lithium, which can limit the maximum possible particle size for a given composition. Depending on the composition, the safety of the charged cathode in a real cell can be a problem. Safety events ultimately are caused by reactions between the oxidized surface and the reducing electrolyte. Thus safety problems are typically more severe if the particles have a high surface area, which is the case if the particle size is small. The conclusion is that the lower performance of LNMCO requires a small particle size which deteriorates safety. It follows that for the “LiNixCoyMnzO2” system, where the presence of given amounts of Ni and Mn are focused both on improving power behaviour and stabilizing the structure, a homogenous composition both for small and large particles leads to a compromise between power and safety performance, due to the unavoidable spread of particle size. Indeed, for the small particles in which safety behaviour is directly related to Mn content, a higher Mn concentration would be needed to achieve the same safety behaviour as for larger particles. On the other hand, the increase of the nickel content in the large particles could enhance the performances of the LiNixCoyMzO2 system. The present invention provides a solution to this problem.